Electronic blackboard and accessories such as writing tools

ABSTRACT

A characteristic and the position of an implement with a tuned circuit having one of plural resonant frequencies are determined. AC energy at the plural different resonant frequencies is supplied to a two-coordinate direction coil arrangement of a position sensing tablet. The tuned circuit changes the current flowing in the coil arrangement at the implement resonant frequency. The current change is used to signal the implement position and characteristic. The implement may be an eraser for supplying a signal to an electronic display and for removing a mark from a surface of a visual display overlaying the tablet of the eraser. A housing includes a surface for erasing the marking and two tuned circuits each having a reactance positioned close to opposite edges of the erasing surface. Two switches, when activated, cause the tuned circuits to have different resonant frequencies. The switches are respectively activated when opposite eraser edges are being pushed against the display surface. The implement may also be one of a plurality of markers, each for a different color. Another display responds to the signals to display the position and colors of the markings and selectively remove markings from areas corresponding to the eraser position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic blackboard for processingimage information which corresponds to the image or the like displayedon the writing surface and its accessories such as writing tools.

2. Prior Art

Hitherto, there is a conventional electronic blackboard apparatus(called "an electromagnetic coupled type" hereinafter) arranged suchthat a multiplicity of sensing lines are, in both x and y directions,formed on the reverse side of the writing surface thereof and a writingtool comprising a felt pen or the like is provided with a coil as togenerate flux change when an electronic current is passed so that thethus-generated flux change is detected by the sensing lines or the otherend of the coil so that the position of the writing tool on the writingsurface is detected and thereby image information which corresponds tothe image displayed on the writing surface can be processed.

Another type of the electronic blackboard apparatus called "aphotoelectric transfer type" hereinafter) is known which is arrangedsuch that an image written on a whiteboard or a flexible seat with awriting tool such as a felt pen or the like is scanned by a scannerwhich can move along the surface of this whiteboard or by a stationaryscanner with this seat wound up as to be photoelectrically transferredto image information.

The other conventional electronic blackboard apparatus (called "apressure sensing type" hereinafter) is known in which two resistanceplates provided with electrodes on the opposing sides thereof arefastened to the reverse side of a writing surface made of a flexiblematerial. The various parts are laminated to each other. These tworesistance plates are fastened to the electrodes in such a manner thatthese electrodes are positioned in the vertical or lateral direction.Therefore, a displacement current is generated between an electrode onone resistance plate and the electrode on the other resistance platewhen the writing tool is moved along the surface of the writing surfacewith an electric current being passed through either of the tworesistance plates. On the basis of the thus-generated displacementcurrent, the position of the writing tool on the writing surface isdetected so that image information corresponding to the image displayedon the writing surface is processed.

However, the above-described electromagnetic coupling type apparatus hasa problem in that, a cord needs to be provided between the control unitfor detecting position and a coil provided for the writing tool. Thethus-provided cord readily deteriorates in handling the writing tool.

In the photoelectrically transfer type apparatus, the writing tool canbe arranged to be a cord-less type. However, another problem arises inthat information corresponding to the displayed image cannot be obtainedduring writing of the image on the writing surface, that is a real-timeimage cannot be obtained since image information can be first obtainedwhen the scanner is moved or the seat is wound up.

In the pressure sensing type apparatus, the writing tool can be arrangedto be a cord-less type and the information corresponding to the imagewhich is being written can be obtained. However, a problem arises inthat the thickness and the weight become excessive since the structureneeds to be formed to withstand the pressure applied with the writingtool to the writing surface.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an electronicblackboard apparatus capable of using a cordless accessary such as aninstruction rod, a marker, or an eraser (called a "writing tool"hereinafter in this specification), obtaining realtime image informationwhich corresponds to the image formed on the writing surface, andexhibiting a thin structure and light weight.

In accordance with one aspect of the invention, an electronic blackboardapparatus comprises a writing surface, in combination with a tool formodifying an image on the surface, wherein the tool includes a tunedcircuit having a predetermined resonant frequency. A sensor for anelectric wave having plural frequencies, one of which is the resonantfrequency, and an electrical wave detector for an electric wavereflected by the tuned circuit is provided. The sensor includescoordinate detection means responsive to the electric waves reflectedfrom the tool and generated by the generating means for detecting acoordinate corresponding to the position of the tool. Image informationprocessing means processes, on the basis of the thus-detectedcoordinate, image information corresponding to an image formed by thetool on the surface.

Another aspect of the invention is directed to the combination of aposition sensing tablet a two coordinate direction coil arrangement, atwo coordinate direction display superposed with the tablet, a markerfor the display and an eraser for the display. The marker and erasereach include a tuned circuit respectively having first and secondresonant frequencies. AC energy is supplied at the first and secondresonant frequencies to the coil arrangement. The tuned circuits on themarker and the eraser, when the marker or eraser is placed in proximityto the tablet and coil arrangement, respectively cause changes incurrents flowing in the coil arrangement at the first and secondfrequencies. The changes in the current flowing in the coil arrangementat the first and second frequencies are sensed. In response to thecurrent changes at the first and second frequencies there is derived asignal indicative of the position of markings by the marker on thedisplay as modified by the eraser.

In accordance with still a further aspect, the invention is directed tothe combination of a position sensing tablet, a two coordinate directioncoil arrangement associated with the tablet and multiple implements formovement relative to the table. Each of the implements has a differentcharacteristic associated therewith and includes a tuned circuit havinga different resonant frequency. AC energy at the different resonantfrequencies is supplied to the coil arrangement. The tuned circuits onthe implements, when the implements are placed in proximity to thetablet and coil arrangement, cause changes in the currents flowing inthe coil arrangement at the different frequencies. Changes in currentflowing in the coil arrangement at the different frequencies are sensed.In response to the current changes at the frequencies there is derived asignal indicative of the position and characteristics of the implementson the tablet.

A further aspect of the invention is directed to a method of identifyinga characteristic and position of an implement on a position sensingtablet having a two coordinate direction coil arrangement associatedtherewith, wherein the implement has one of plural characteristics andincludes a tuned circuit having one of plural different resonantfrequencies. The method comprises supplying AC energy at the pluraldifferent resonant frequencies to the coil arrangement. The tunedcircuit on the implement, when placed in proximity to the tablet andcoil arrangement, causes changes in current flowing in the coilarrangement at the one frequency. Changes in the current flowing in thecoil arrangement at the different frequencies are sensed. In response tocurrent changes at the frequencies there is derived a signal indicativeof the position and characteristic of the implement on the tablet.

Still another aspect of the invention is directed to an eraser forsupplying a signal to an electronic display and for removing a markingfrom a surface of a visual display. The eraser comprises a housingincluding: a surface for erasing the marking, first and second tunedcircuits each having a reactance positioned in proximity to first andsecond opposite edges of the erasing surface, as well as first andsecond switches which when activated respectively cause the first andsecond tuned circuits to have different first and second resonantfrequencies. The first and second switches are positioned and arrangedso that the first and second switches are respectively activated inresponse to the first and second edges of the eraser being pushedagainst the display surface.

Still a further aspect of the invention is directed to the combinationof a position sensing tablet, a two-coordinate direction coil, atwo-coordinate direction display superposed with the tablet, andmultiple implements for modifying markings of the display. Eachimplement has a different characteristic associated therewith andincludes a tuned circuit having a different resonant frequency for eachcharacteristic. AC energy at the different resonant frequencies issupplied to the coil arrangements. The tuned circuits on the implements,when the implements are placed in proximity to the tablet and coilarrangement, respectively cause changes in the currents flowing in thecoil arrangement at the different frequencies. Changes in the currentflowing in the coil arrangement at the different frequencies are sensed.In response to current changes at the frequencies there is derived asignal indicative of the position and characteristics of markings by theimplements on the display.

Still a further aspect of the invention is directed to the combinationof a position sensing tablet, a two-coordinate coil arrangementassociated with the tablet, a two-coordinate direction displaysuperposed with the tablet and plural markers for the display. Each ofthe markers is for a different color on the display. Each of the markersincludes a tuned circuit having a different resonant frequency. ACenergy at the different resonant frequencies is supplied to the coilarrangement. The tuned circuits on the markers, when the markers areplaced in proximity to the tablet and coil arrangement, cause changes incurrents flowing in coils of the coil arrangement at the differentfrequencies. In response to energy coupled between the tablet andmarkers there is derived a signal indicative of the position of thecolors of markings by the markers on the display by sensing changes incurrent flowing in the coil arrangement at the different frequencies.

Yet an additional aspect of the invention is directed to a method ofdisplaying a polychromatic image by marking a first two-coordinatedirection display with plural markers each having a different color anda tuned circuit with a different resonant frequency thereon. Pluralfrequencies are supplied to a two-coordinate direction coil arrangement.The coil arrangement couples the plural frequencies to each marker as itis marking the first display. The coil arrangement is activated so it isresponsive to an interaction of each applied frequency and each tunedcircuit to provide an indication of the color and position of the markbeing made by each marker on the first display. By responding to theindication there is displayed on a second two-coordinate directiondisplay the position and color of the mark made by each marker on thefirst display.

Another aspect of the invention concerns a method of indicating theposition of an image on a two-coordinate direction display by markingthe display with a marker and erasing from the display at least aportion of marks made by the marker. The marker and eraser respectivelyhave tuned circuits with first and second resonant frequencies. Atwo-coordinate direction coil arrangement is activated so it applies thefirst and second resonant frequencies to the marker and the eraser asthey respectively mark and erase the display. The coil arrangement isactivated to be responsive to an interaction of the applied first andsecond frequencies with the tuned circuits to derive an indication ofthe position of the marking on the display as modified by the eraser.

Still an additional aspect of the invention is directed to an eraser forsupplying a signal to an electronic display and for removing a markingfrom a surface of a visual display. The eraser has a housing including:a surface for erasing the marking, a tuned circuit having a reactancepositioned immediately behind the eraser surface, and switch meansactivated in response to the erasing surface being pressed against thedisplay surface for connecting elements including the reactance of thetuned circuit together so they have a predetermined resonant frequencywhile the erasing surface is pressed against the display surface.

A second object of the present invention is to provide an electronicblackboard apparatus capable of identifying the type and the state ofthe writing tool which is being used and to thereby obtain imageinformation which corresponds to the thus-identified type or state ofthe writing tool.

A third object of the present invention is to provide an electronicblackboard apparatus including two writing surfaces.

A fourth object of the present invention is to provide a writing toolsuch exhibiting a simple structure, light weight, and easy handlingcapability, that is, an instruction rod, a marker, and an eraser.

Other objects and features of the present invention will become moreapparent in the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a first embodiment of an electronic blackboardapparatus according to the present invention;

FIG. 2 is a structural view of loop coil groups in X and Y directions ofa sensing portion;

FIG. 3 is a cross-sectional view of an instruction rod;

FIG. 4 is a cross-sectional view of a marker;

FIG. 5 is a cross-sectional view of an eraser;

FIG. 6 is a block diagram of the electronic blackboard apparatusaccording to the present invention;

FIG. 7 is a schematic block diagram of tuning circuits for a writingtool and details of a control unit for a sensing portion;

FIG. 8 is a series of waveforms used for describing the circuit of FIG.7;

FIG. 9 is a timing diagram used to describe the circuit of FIG. 7;

FIG. 10 is a flow chart of the coordinate-detection action performed bya control unit of the sensing portion;

FIGS. 11A, 11B, and 11C are further waveforms used for describingcoordinate detection action performed by the control unit of the sensingportion;

FIG. 12 is a waveform of the levels of the detected voltage obtainablefrom each of the loop coils when coordinate detection action isperformed;

FIG. 13 is a block diagram of a data processing unit used with thepresent invention;

FIG. 14 is a flow chart for processing of image information performed bythe data processing unit;

FIG. 15 is a view of a second embodiment of the electronic blackboardapparatus according to the present invention;

FIG. 16 is a view of a partial cross-sectional view of a frame accordingto the second embodiment;

FIG. 17 is a flow chart of a program for the data processing unitaccording to the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a view of a first embodiment of an electronic blackboardapparatus according to the present invention, wherein reference numeral1 represents a main blackboard body, 2 represents an instruction rod, 3represents a marker, 4 represents an eraser, and 5 represents a controlbox.

The main blackboard body 1 is formed such that legs 13 are fastened to aframe 12 a portion of which corresponds to the front surface of asensing portion 11 is made of a non-metallic material having a flatsurface so as to form a writing surface 14 which can be repeatedly usedwith the marker 3 and the eraser 4. Alternatively, the portion of theframe 12 which corresponds to the front surface of the sensing portion11 may be formed by applying paint to which magnetic dust is mixed tothe plate made of the non-metallic material or by laminating a filmsheet such as a magnet sheet and a plastic sheet on the same plate forthe purpose of realizing a writing surface to which sheet in whichintended frames or figures are written can be temporally fastened bymetal pieces or magnets.

In FIG. 2 are illustrated a loop coil group 11x in x-direction and aloop coil group 11y in y-direction which form the sensing portion 11.The loop coil group 11x in x-direction comprises a multiplicity of, forexample, 48 loop coils 11x-1, 11x-2, . . . , 11x-48 so as to be arrangedin parallel to each other and to overlap each other. The loop coil group11y in y-direction comprises a multiplicity of, for example, 30 loopcoils 11y-1, 11y-2, . . . , 11y-30 so as to be arranged in parallel toeach other and to overlap each other. The loop coil group 11x inx-direction and the loop coil group 11y in y-direction overlap eachother with positioned closely contact with each other (in order toreadily understand the structure, they are drawn in a separated manner).Alternative to the structure employed here in which each of the loopcoils is formed by one turn, the loop coils may be formed by a pluralityof turns if necessary.

In FIG. 3 is illustrated a first embodiment of the writing tool and soon according to the present invention, in which the structure of theinstruction rod 2 is illustrated. This instruction rod 2 is made of asynthetic resin, wood or the like and is formed in such a manner that atuning circuit 22 comprising a coil 221 which includes a core and acapacitor 222 is accommodated in a recessed portion 21a formed at thefront end portion of a column-like main body 21 made of a syntheticresin or wood in such a manner that the axis of this coil 221substantially meets the longitudinal direction of the main body 21(frame), and this recessed portion 21a is covered with a cap 23 made ofthe similar material to that for the main body 21. The main body 21 maycomprise a telescopic rod.

The coil 221 and the capacitor 222 are, as shown in FIG. 7, connected toeach other in series so as to form a known resonant circuit. Theinductance of this coil 221 and the capacity of the capacitor 222 aredetermined as to make the resonant (tuned) frequency thereofsubstantially a predetermined frequency f0, for example, 600 kHz.

FIG. 4 is a view of a second embodiment of the writing tool according tothe present invention, in which the structure of a marker 3 is in detailillustrated. This marker 3 comprises: a pen shaft 31 formed by twoportions 31a and 31b made of a non-metallic material such as a syntheticresin and to be screw-coupled with each other; a pen body 32 such as ablack felt pen on the market or the like; a push switch 331; a coil 332including a core; a tuning circuit 33 comprising capacitors 333 and 334;and a cap 34 for the pen body 32.

The pen body 32 is accommodated in a space formed by a stopper 31a'formed in the portion 31a of the pen shaft 31 and the switch 331accommodated in the portion 31b such that the same can be slightly movedtherein. The coil 332 is accommodated in the portion at the front end ofthe portion 31a of the pen shaft 31 such that the axial directionthereof is substantially coincident with the longitudinal direction ofthe pen shaft 31 (frame).

As shown in FIG. 7, the coil 332 and the capacitor 333 are connected toeach other in series so as to form a known resonant circuit. Theinductance of this coil 332 and the capacity of the capacitor 333 are sodetermined as to make the resonant (tuned) frequency thereofsubstantially a predetermined frequency f0, for example, 600 kHz. Thecapacitor 334 is in parallel connected to the two ends of the capacitor333 via the switch 331 so that the same acts to change the tunedfrequency in the resonant circuit from the predetermined frequency f0 toanother frequency f1, for example, 550 kHz when the above-describedswitch 331 is switched on.

This switch 331 is arranged to be switched on when it is abutted by therear end of the pen body 32 which has been pushed into the pen shaft 31as a result of abutting the front end of the pen body 32 against thewriting surface 14 or the like with the pen shaft 31 held by the hand orthe like.

Although the front end of the pen body is made project over the writingsurface as to enable the writing according to this embodiment, it may bearranged to be capable of projecting only when used. The marker is sodesigned that the pen body 32 included therein can be replaced by soarranging the pen shaft 31 that it can be divided and coupled to eachother.

In FIG. 5 is illustrated a third embodiment of the writing tool or thelike according to the present invention, in which the structure of theeraser 4 is illustrated in detail. This eraser 4 comprises: a case 41made of a non-metallic material such as a synthetic resin or the like; amovable plate 42 comprising a pair of members 42a and 42b each of whichhas a shape corresponding to the bottom surface of the case 41 and whichare arranged to be detachable to each other; a pair of springs 43a and43b inserted between the movable plate 42 and the case 41; a stopper 44disposed in an inner portion 41a of the case 41 and capable ofsupporting a securing portion 42b' which projects in the direction ofthe member 42b perpendicular to the drawing sheet for the purpose ofrestricting the position of the movable plate 42 with respect to thecase 41; an erasing member 45 made of felt or the like and fastened tothe outer surface of the member 42a of the movable plate 42; a firsttuning circuit 46 comprising a switch 461 accommodated in the innerportion 41a of the case 41, a capacitor 462 and a capacitor 463 whichincludes a core and is held between the members 42a and 42b at theposition corresponding to the spring 43a of the movable plate 42 suchthat the axial direction thereof and the erasing surface formed by theerasing member 45 are substantially perpendicular to each other; and asecond tuning circuit 47 comprising a switch 471, a capacitor 472, and acoil 473 which includes a core and is held between the members 42a and42b at the position corresponding to the spring 43b such that the axialdirection thereof and the erasing surface formed by the erasing member45 are substantially perpendicular to each other. This movable plate 42is held such that the same can move slightly with respect to the case 41so that the switch 463 and/or 473 can be operated.

The coil 463 and the capacitor 462 are connected to each other in seriesvia the switch 461 as shown in FIG. 7 so that a known resonant circuitis actuated when this switch 461 is switched on. The inductance of thiscoil 463 and the capacity of the capacitor 462 are determined as to makethe resonant (tuned) frequency thereof substantially another frequencyf2, for example, 500 kHz.

The coil 473 and the capacitor 472 are connected to each other in seriesvia the switch 471 as shown in FIG. 7 so that a known resonant circuitis actuated when this switch 471 is switched on. The inductance of thiscoil 473 and the capacity of the capacitor 472 are determined as to makethe resonant (tuned) frequency thereof substantially other frequency f3,for example, 450 kHz.

These switches 461 and 471 are switched off when the eraser 4 is notoperated, while either or both of the switches 461 and 471 are switchedon by being pressed by the member 42b of the movable plate 42 when theerasing member 45 of the movable plate 42 is pushed into the case 41 byabutting this erasing member 45 against the writing surface 14 or thelike with the case 41 held by the hand or the like.

The tuning circuit for the writing tool or the like of the typedescribed above is so arranged that it can be synchronized with theenergy of the electric wave discharged from the electric wave generatingmeans in the sensing portion and discharge this energy to the electricwave detection means in the sensing portion for the purpose of meetingthe conditions required to perform indication of the position to bemeasured to the sensing portion.

The control box 5 is provided with, on the front surface thereof,various control switches 51 and an outlet 52 through which printedsheets are discharged, this control box 5 including, as shown in FIG. 6,the sensing portions control unit 6, the data processing unit 7, and aprinter 8.

In FIG. 7 is illustrated the sensing portion control unit 6 togetherwith the instruction rod 2, the marker 3, the tuning circuits 22, 33,46, and 47 for the eraser 4. Referring to this drawing, referencenumeral 601 represents a control circuit, 602 represents a signalgenerating means (circuit), 603x and 603y respectively representselection means (circuit) in x-direction and y-direction, 604x an 604yrepresent transmission and reception switch circuits, 605 represents aXY switch circuit, and 606 represents a reception timing switch circuit,whereby connection switch means is formed. Reference numeral 607represents a BPF (Band-Pass Filter) which forms a signal detectionmeans. Reference numeral 608 represents a detector and 609 represents anLPF (Low-Pass Filter) which form coordinate detection means and writingtool or the like identification means in which a process performed bythe control circuit 601 to be described later is included. Referencenumerals 610 an 611 represent drive circuits, 612 and 613 representamplifiers, and 614 represents an inverter.

The operation of the sensing portion control unit 6 with the structurethereof will be described. First, the signal transmission and receiptbetween the sensing portion 11 and the writing tool or the like and thethus-obtained signals will be described with reference to FIG. 8.

The control circuit 601 comprises a known type of a microprocessor orthe like. It acts to: supply a frequency switch signals p1 and P2(quadrual counter data) and a timing signal (start pulse) p3 to thesignal generating circuit 602 in accordance with a flow chart to bedescribed later: control the switching of the loop coils in the sensingportion 11 via the selection circuits 603x and 603y; control theswitching of the coordinate detection direction performed by the XYswitch circuit 605 and the reception timing switch circuit 606;analog-digital (A/D) convert the output data from the low pass filter609 for the purpose of obtaining the coordinate of the writing tool orthe like by performing calculations to be described later; and supplythe frequency switch signals p1 and p2 when the above-describedcoordinate is obtained to the data processing unit 7, these frequencyswitch signals p1 and p2 serving information representing the type orthe state (a state of the switch for the marker 3) of the writing toolor the like.

The signal generating circuit 602 comprises a rectangular signalgenerators 602a, 602b, 602c, 602d, and 602e, and a multiplexer 602f forrespectively generating predetermined frequencies f0, f1, f2, f3, andfk.

The rectangular signals having frequencies from f0 to f3 are arranged tobe input to the multiplexer 602f whose switching is controlled inresponse to the switch signals p1 and p2. When the switching signals p1and P2 are "00" , the signal having the frequency f0 is output, when thesame are "01", the signal having the frequency fl is output, when thesame are "10", the signal having the frequency f2 is output, and whenthe same are "11", the signal having the frequency f3 is output, thethus-generated signal being output in the form of a rectangular signalA. The thus-output rectangular signal A is converted to a sine-wavesignal by a low-pass filter (omitted from illustration), and is thensupplied to either the drive circuit 610 or 611 via the XY switchcircuit 605.

A rectangular signal having the frequency fk, for example, 18.75 kHz, istransmitted to the transmission and reception switch circuits 604x and604y in the form of a transmission and reception switch signal B, and issimultaneously inverted via the inverter 614 as to be transmitted to thereception timing switch circuit 606 in the form of a reception timingsignal C. The rectangular signal generator 602e is reset by the startpulse p3.

The selection circuit 603x successively selects a loop coil from thex-direction loop coil group 11x, while the selection circuit 603ysuccessively selects a loop coil from the y-direction loop coil group11y, each of these selection circuits 603x and 603y acting in responseto information supplied from the control circuit 601.

The transmission and reception switch circuit 604x alternately connectthe thus-selected x-direction loop coil to the drive circuit 610 and theamplifier 612. The transmission and reception switch circuit 604yalternately connect the thus-selected y-direction loop coil to the drivecircuit 611 and the amplifier 613. These transmission and receptionswitch circuits 604x and 604y act in accordance with the transmissionand receipt switch signal B.

It is assumed that "00" has been, together with the start pulse p3,supplied from the control circuit 601 to the signal generating circuit602 in the form of the switch signals p1 and p2 and information toselect x-direction has been input to the XY switch circuit 605 and thereception timing switch circuit 606, the sine-wave signal having thefrequency f0 is supplied to the drive circuit 610 in which it isconverted to an equilibrium signal before being supplied to thetransmission and reception switch circuit 604x. Since this transmissionand reception switch circuit 604x switches and connects either of thedrive circuit 610 or the amplifier 612 in response to the transmissionand reception switch signal B, a signal to be output from thetransmission and reception switch circuit 604x to the selection circuit603x becomes a signal D which intermittently outputs a sine-wave signal600 kHz every time period T (=1/2 fk), where it is substantially 27 μsechere.

This signal D is transmitted to the x-direction loop coil 11x-i (i=1, 2,. . . , 48) in the sensing portion 11 via the selection circuit 603x,this loop coil 11x-i generating an electric wave on the basis of thesignal D.

In this state, when the writing tool, for example, the marker 3 is heldsubstantially vertically on the writing surface 14 of the mainblackboard body 1 with the switch 331 switched off, the above-describedelectric wave excites the coil 332 of the marker 3 so that an inducedvoltage E synchronized with the signal D is generated in the tuningcircuit 33 of the coil 332.

When the state of the signal D is then brought to a no-signal period,that is, signal reception period and simultaneously the loop coil 11x-iis switched to the amplifier 612, the electric wave from this loop coil11x-i is immediately eliminated. On the other hand, the above-describedinduced voltage E is gradually damped in accordance with loss in thetuning circuit 33.

On the other hand, the electric current passing through the tuningcircuit 33 in accordance with this induced voltage E causes the coil 332to transmit an electric wave. Since the thus-transmitted electric waveexcites the loop coil 11x-i connected to the amplifier 612 on thecontrary, an induced voltage on the basis of the electric wave from thecoil 332 is generated. The thus-generated induced voltage is transmittedfrom the transmission and reception switch circuit 604x to the amplifier612 during only the signal reception period so that it is amplified tobecome a reception signal F, and is then transmitted to the receptiontiming switch circuit 606.

Either of the selection information in x-direction or y-direction, thex-direction selection information and the reception timing signal C inthis case, are input to the reception timing switch circuit 606. Whenthis signal C is at the high (H) level, a reception signal F is output,while no signal is output when the same is at the low (L) level.Therefore, a signal G (substantially the same as the reception signal F)is obtained at the output of the reception timing switch circuit 606.

Since this signal F is transmitted to the band-pass filter 607 which isa filter including the frequency f0 to f3 in its band-pass region, asignal H (strictly, in the state in which a plurality of signals G havebeen input to and converged in the band-pass filter 607) having anamplitude h in accordance with energy of the frequency components fromf0 to f3 in the above-described signal G transmitted to the detector608.

The signal H input to the detector 608 is detected and rectified as tobe made a signal I. Then, this signal I is converted into a directcurrent J having a voltage level corresponding to a half of theabove-described amplitude h, for example, Vx by a low-pass filter 609with a sufficiently low cut-off frequency as to be transmitted to thecontrol circuit 601.

The voltage level Vx of the signal J relates to the distance between themarker 3 and the loop coil 11x-i, where it is a value in inverseproportion to substantially the fourth power of the distance between themarker 3 and the loop coil 11x-i. Therefore, when the loop coil 11x-i isswitched, this voltage Vx of the signal J is varied. As a result, thex-coordinate of the marker 3 can be obtained by converting, in thecontrol circuit 601, the voltages Vx obtained for each of the loop coilsinto digital values and by having the thus-obtained digital valuessubjected to the arithmetic process to be described later. They-coordinate of the marker 3 can be obtained similarly.

On the other hand, when the marker 3 and the writing surface 14, that isthe sensing portion 11, are disposed away from each other, or when themarker 3 is disposed substantially in parallel to the sensing portion11, the electric wave transmitted from the loop coil in the sensingportion 11 does not excite the coil 332 of the marker 3. Therefore, noinduced voltage E is generated in the turned circuit 33. In this state,since also no electric wave is transmitted from the coil 332 of thetuning circuit 33, no induced voltage F is generated in the loop coil ofthe sensing portion during signal reception so that the coordinatecannot be detected (practically, a slight level of induced voltage isgenerated in both the tuned circuit and the loop coil in the sensingportion, their levels are insufficient to perform the coordinatedetection). The above-described frequency switch signals p1 and p2 arethe values counted by the quadrual ring counter formed by a program orthe like in the control circuit 601. This counter is stepped to "1" whenno reception signal, that is, no induced voltage is obtained in thecontrol circuit 601 and the detection of the coordinate is therebyimpossible to be performed. The value counted at this time is, togetherwith the start pulse p3, arranged to be transmitted in the form of theswitch signals p1 and p2 to the signal generating circuit 602.Therefore, during the period in which no reception signal is obtained,the frequency of the AC signal is successively switched from f0 to f3 sothat the detection of the coordinate is performed by repeating thisswitching of the frequency.

If any reception signal is obtained, the x and y-coordinates can beobtained as described above. At this time, if the switch signal p1 andp2 are "00" or "01", that is if the frequency of the AC signal is f0 orf1, the above-described counter is not stepped so that the frequenciesof the switch signals p1 and p2, that is the frequency of the AC signalis maintained intact. On the other hand, if the switch signal p1 and p2are "10" or "11", that is if the frequency of the AC signal is f2 or f3,the above-described counter is stepped by "1" so that the frequencies ofthe switch signals p1 and p2, that is the frequency of the AC signal issuccessively switched.

As described above, when the tuning circuit 22 and 20 the switch 331 ofthe instruction rod 2 are turned off, the tuned frequency of the tuningcircuit 33 of the marker 3 is f0, when the switch 331 is switched on,the tuned frequency of the tuning circuit 33 of the marker 3 is f1, whenthe switch 461 is switched on, the tuned frequency of the tuning circuit46 of the eraser 4 is f2, and when the switch 471 is switched on, thetuned frequency of the tuning circuit 47 of the eraser 4 is f3.Therefore, if the switch signals p1 and p2 representing the frequency ofthe AC signal are "00" when the reception signal can be obtained, a factcan be detected that the apparatus is used such that the instruction rod2 or the pen body 32 of the marker 3 is not positioned in contact withthe writing surface 14 and thereby the switch 331 is switched off. Ifthe switch signals p1 and p2 are "01", a fact can be detected that theapparatus is used such that the pen body 32 of the marker 3 ispositioned in contact with the writing surface 14 and thereby the switch331 is switched on, that is, a fact can be detected that image is beingwritten on the writing surface 14. If the switch signals p1 and p2 are"10" or "11", a fact can be detected that the apparatus is used suchthat the erasing member 45 of the eraser 4 is positioned in contact withthe writing surface 14 and thereby the switch 416 or 471 is switched on,that is, a fact can be detected that the image on the writing surface isbeing erased.

Therefore, the switch signals p1 and p2 representing the frequencies ofthe AC signal when the above-described reception signal is obtainedserve identification information representing the type or the state ofuse of the writing tool which is being used on the writing surface 14.

FIG. 9 is a timing diagram of an example of transition of the switchsignals p1 and p2. First, when the writing tool or the like ispositioned away from the writing surface 14, the switch signals p1 andp2 are successively switched as "00", "01", "10", and "11". When thewriting tool or the like, for example, the marker 3 is allowed to comecloser to the writing surface 14 with substantially erected, thecoordinate is detected by the AC signal having the frequency f0, causingthe AC signal having the frequency f0 to be generated repeatedly. Then,when the pen body 32 is brought into contact with the writing surface 14(brought to a pen down state) between the time point t3 and t4, that is,when the switch 331 is switched on, the coordinate detection by means ofthe AC signal having the frequency f1 is repeatedly performed.Furthermore, when the pen body 32 of the marker 3 is moved away from thewriting surface 14 (brought to a pen up state), that is, when the switch331 is switched off, the coordinate detection by means of the AC signalhaving the frequency f1 is stopped. Then, a transition to frequency f2and f3 is, similarly to the above-description, realized.

The data of the switch signals p1 and p2 are, together with the obtainedx and y-coordinate data, supplied to the data processing unit 7.

As described above, when the switch signals p1 and p2 representing thefrequency of the AC signal are "00" or "01" at the time of obtaining thereception signal, the frequency of the switch signal, that is, thefrequency of the AC signal is maintained intact. The reason for thislies in that the cycle of detecting the coordinate when the instructionrod 2 or the marker 3 is used is intended to be shortened as possiblefor the purpose of improving following-up performance. On the otherhand, when the switch signals p1 and p2 representing the frequency ofthe AC signal are "10" or "11" at the time of obtaining the receptionsignal, the frequency of the switching signal, that is, the frequency ofthe AC signal is successively switched. The reason for this lies in thattwo tuning circuits 46 and 47 having individual frequencies aresometimes operated and their coordinates thereby need to besimultaneously detected when the eraser 4 is used. In addition, thesignificantly excellent following-up capability is not needed withrespect to the marker 3 or the like which writes image. The structuremay be arranged such that the signals f0 to f3 are always and repeatedlygenerated regardless of the results of the coordinate detection althoughthe coordinate-detection speed is slightly reduced.

Then, the operation of the sensing portion control unit 6 will be indetail described with reference to FIGS. 10 to 12.

The control circuit 601 resets the above-described quadrual counter(step sp1), transmits the thus-obtained counter data, that is, theswitch signals p1 and p2 with the start pulse p3 to the signalgenerating circuit 602 (step sp2). transmits information for selectingx-direction to the XY switch circuit 605 and the transmission andreception switch circuit 606, transmits information for selecting thefirst loop coil 11x-1 from the x-direction loop coils from 11x-1 to11x-48 in the sensing portion 11 to the selection circuit 603x, andconnects the thus-selected loop coil 11x-1 to the transmission andreception switch circuit 604x.

The transmission and reception switch circuit 604x alternately connectsthe loop coil 11x-1 to the drive circuit 610 and the amplifier 61 inresponse to the above-described transmission and reception switch signalB. At this time, the drive circuit 610 transmits 16 sine wave signals of600 kHz as shown in FIG. 11A to the loop coil 11x-1 during the signalreception time period of substantially 27 μsec.

The above-described switching between signal transmission and signalreception are, as shown in FIG. 11B, repeated 7 times for one loop coil,where it is 11x-1. The time period in which the signal transmission andsignal reception are repeated 7 times corresponds to the selectionperiod for one loop coil.

At this time, an induced voltage can be obtained at the output of theamplifier 612 for one loop coil every reception time period of 7 times.The thus-obtained induced voltages are, as described above, transmittedto the band-pass filter 607 via the reception timing switch circuit 606,wherein the same is averaged and then is transmitted to the controlcircuit 601 via the detector 608 and the low-pass filter 609.

The control circuit 601 inputs the output value from the above-describedlow-pass filter 609 after A/D converting the same as to store the sameas the detected voltage related to the distance between the writing toolor the like and the loop coil 11x-1, for example as Vx1.

Then, the control circuit 601 transmits information for selecting theloop coil 11x-2 to the selection circuit 603x, connects this loop coil11x2 to the transmission and reception switch circuit 604x, obtains andstores the detection voltage Vx2 relating to the distance between thewriting tool or the like and the loop coil 11x-2, successively andsimilarly connects the loop coils 11x-3 to 11x-48 to the transmissionand reception switch circuit 604x, and stores the detection voltages Vx1to Vx48 (however, FIG. 11C illustrates only a part of the voltages in ananalog-like manner) relating to each of the distances between each ofthe loop coils as shown in FIG. 11C and the writing tool or the like inx-direction (step sp3).

The practical detected voltages are, as shown in FIG. 12, obtained inseveral loop coils centering the position (xp) of the writing tool.

Then, the control circuit 601 transmits y-direction selectioninformation to the XY switch circuit 605 and the reception timing switchcircuit 606, similarly switches the selection circuit 603y and thetransmission and reception switch circuit 604y, and temporally storesthe detected voltage relating to each of the distances between thewriting tool or the like and each of the loop coils 11y-1 to 11y-30 iny-direction and obtained by A/D-converting the output value from thelow-pass filter 609 (step sp4).

Then, the control circuit 601 determines whether or not the level of thedetected voltage which has been stored exceeds a predetermined level(step sp5). If it is below the predetermined level, the quadrual counteris stepped by "1" (step sp6), and the above-described steps sp2 to sp5are repeated. If the same exceeds the predetermined level, the x andy-coordinates of the writing tool or the like are calculated from thethus-stored voltage level in a manner to be described later (step sp7),transmits the thus-calculated coordinates with the switch signals p1 andp2 to the data processing unit 7 (step sp8), and determines whether ornot the switching signals p1 and p2 is "00" or "01" at this time (stepsp9). If the same are "00" or "01", the process according to steps sp2to sp9 are repeated with the quadrual counter maintained intact. If thesame are "10" or "11", the quadrual counter is stepped by "1" (stepsp6), and the processes according to steps sp2 to sp9 are repeated.

As a method for calculating the x or y-coordinate, for example, theabove-described coordinate xp, there is a method in which the waveformsin the vicinity of the maximal values of the above-described detectedvoltages Vx1 to Vx48 are approximated by an appropriate function and thecoordinates of the maximal value of this function are calculated.

For example, referring to FIG. 11C, when the detected voltage Vx3 of themaximal value and the detected voltages Vx2 and Vx4 disposed on bothsides of the former are approximated by a quadratic function, thecoordinates can be calculated as follows (where it is provided that thecoordinates of the central position of each of the loop coils 11x-1 to11x-48 are x1 to x48 and the distances between the central positions areΔx): first, from each of the voltages and the coordinates,

    Vx2=a(x2-xp).sup.2 +b                                      (1)

    Vx3=a(x3-xp).sup.2 +b                                      (2)

    Vx4=a(x4-xp).sup.2 +b                                      (3)

where a and b represent constants (a>0). Furthermore, the followingequations holds:

    x3-x2=Δx                                             (4)

    x4-x2=2Δx                                            (5)

Substituting Equations (4) and (5) into Equations (2) and (3) beforerearrangement, the following equation holds:

    xp=x2+Dx/2 {3Vx2-4Vx3+Vx4)/(Vx2-2Vx3+Vx4)}                 (6)

Therefore, the coordinate xp of the writing tool or the like can becalculated by performing the calculation expressed in Equation (6) byusing the detected voltage of the maximal value and the detectedvoltages in the vicinity of this maximal value which have been obtainedat the above-described level check extracted from the detected voltagesVx1 to Vx48 and the coordinates (known) of the loop coil which isdisposed forward by one from the loop coil at which the detected voltageof the above-described maximal value has been obtained.

FIG. 13 is a block diagram of the data processing unit 7, whereinreference numeral 71 represents a microprocessor (CPU), 72 represents aframe memory, 73 represents a overlay memory, 74, 75, 76, and 77represent interface circuits which respectively corresponds to theoperation switch 51, sensing portion control unit 6, printer 8, anddisplay (omitted from illustration).

FIG. 14 is a flow chart of the program relating to processing of imageinformation in the data processing unit 7. Image information processingmeans is formed by this program and the microprocessor 71.

The operation of the data processing unit 7 will now be described.

The microprocessor 71 receives data comprising the x and y-coordinatesand the identification information from the sensing portion control unit6 via the interface circuit 75 (step s1), and determines whether or notthe thus-received information is "00" (step s2).

If the identification information is "00", the microprocessor 71determines that the coordinates at this time are temporal positionaldata, and causes the character generator (omitted from illustration) togenerate a cursor pattern ,for example, an arrow "↑" as to be written inan address in the overlay memory 73 which corresponds to theabove-described coordinates (step s3). Since the contents of the overlaymemory 73 can be lost if no data is written within a predetermined timeperiod (usually several ms), the above-described address in which thecursor has been written is changed in accordance with the change of thecoordinates transmitted from the sensing portion control unit 6.

If the identification information is not "00", it is determined whetheror not the same is "01" (step s4). If the identification information is"01", it is determined that the data corresponds to the image drawn onthe writing surface 14 by the marker 3 so that bit "1" is written in theaddress in the frame memory 72 corresponding to the above-describedcoordinates (step s5). Contents written in the frame memory 72 can beretained if no other data is written therein.

If the identification information is neither "00" nor "01", it isdetermined that the data is the data for determining a predeterminedrange on the writing surface 14 to be erased by the eraser 4 so that bit"0" is written in the address in the frame memory 72 corresponding tothe predetermined range to be erased and determined by theabove-described coordinates (step s6), and the image information isdeleted.

When a printing-out switch (omitted from illustration) of theabove-described operation switches 51 is operated, the microprocessor 71transmits the contents of the frame memory 73 to the printer 8 via theinterface circuit 76 so that these contents are subjected to a hardcopying process.

When a display device is connected, the microprocessor 71 simultaneouslyreads out the contents of the frame memory 72 and the overlay memory 73via the interface circuit 77 and simultaneously converts the contents toa video signal as to be transmitted to and displayed on the displaydevice.

When the marker 3 or the eraser 4 is used along the writing surface 14,the contents of the frame memory 72 are rewritten in accordance with thecoordinates at that time. When the instruction rod 2 is used on thewriting surface 14, or when the marker 3 is used such that the same doesnot come contact with the writing surface 14, the cursor is written inthe overlay memory 73 in accordance with the coordinates at that time.Therefore, image corresponding to the image written on the writingsurface 14 is displayed by dots on the frame of the display device, andthe position of the instruction rod 2 or the marker 3 pointing anoptional position on the writing surface 14 is indicated by the cursor.

Furthermore, data received from the sensing portion control unit 6 canbe transmitted to the other electronic blackboard connected by means ofan interface circuit, a MODEM and communication lines for the purpose ofdisplay the similar image or cursor on the display device of the otherelectronic blackboard.

As an alternative to the above-described embodiment in which only onetype of marker comprising a black felt pen is used, a multiplicity ofmarkers comprising the other color felt pens, for example, red, blue andso on and tuning circuits each of which having individual frequenciesmay be prepared, these markers being identified from the above-describedidentification information as to be processed on frame memoriescorresponding to the multiplicity of colors.

FIG. 15 is a diagram of a second embodiment of the electronic blackboardaccording to the present invention, in which an example of a structurein which two writing surfaces are provided is illustrated. Referring tothis drawing, reference numeral 15 represents a frame supported as to berotatable with respect to the legs 16 with a support shaft 17. Each ofthe obverse side and the reverse side of this frame 15 can be optionallymade face the direction of the surface of this drawing sheet.

The frame 15 is, as shown in FIG. 16, provided with, on both sides ofthe sensing portion 11 thereof, honeycomb members 151 and 152 made of anon-metallic material such as a synthetic resin or the like, and boards153 and 154 which are similarly made of a non-metallic material aredisposed on both sides of the above-described honeycomb members 151 and152. The boards 153 and 154 respectively have corresponding writingsurfaces 18 and 19 which can be repeatedly used.

The frame 15 and the legs 16 respectively includes couplers 91, 92, and93 for the purpose of transmitting information therebetween. Thesecouplers 91 to 93 are arranged such that when either of the two writingsurfaces, for example, the writing surface 18 is made face the surfaceof this drawing sheet, the couplers 91 and 92 confront each other, whilewhen the other one, that is, the writing surface 19 is made face thesurface of this drawing sheet, the couplers 91 and 93 confront eachother.

These couplers 91 to 93 include, for example, light emitting diodes orphototransistors for the purpose of transmitting information by means ofoptical signals. According to this embodiment, the sensing portioncontrol unit 6 is disposed within the frame 15 (for example, in thecircumferential portion of the sensing portion 11) so that informationto be transmitted by means of the couplers 91 to 93 becomes theabove-described coordinates or identification information.

A metallic member 94 and non-contact sensors 95 and 96 are respectivelydisposed adjacent to the above-described positions at which the couplers91 to 93 are positioned in the frame 15 and the legs 16 so that when thewriting surface 18 is made face the surface of this drawing sheet, themetallic member 94 and the non-contact sensor 95 confront each other,while when the other one, that is, the writing surface 19 is made facethe surface of this drawing sheet, the metallic member 94 and thenon-contact sensor 96 confront each other so that detection of the factwhich one of the writing surfaces 18 and 19 is being positioned to facethe surface of this drawing sheet can be readily performed, that is, afact that which one is being used can be readily detected. The outputsfrom the non-contact sensors 95 and 96 are transmitted to the dataprocessing unit 7 via an interface circuit (omitted from illustration).

The power for the sensing portion control unit 6 in the frame 16 isarranged to be supplied through a mechanical and electric contact(omitted from illustration) disposed similarly to the couplers 91 to 93.

According to the above-described apparatus, the vertical positions ofthe sensing portions 11 are made inverse between the case in which thewriting surface 18 is made to face the surface of this drawing sheet andthe case in which the writing surface 19 is made to face the same.Therefore, even if the same image has been written, coordinates whosevertical positions are different are output from the sensing portioncontrol unit 6.

FIG. 17 is a flow chart of a program employed in the data processingunit 7 according to the present invention. When data from the sensingportion control unit 6 is received, a fact that which one of the writingsurface of the frame 15 faces the surface of this drawing sheet isdetected on the basis of the outputs from the non-contact sensors 95 and96 (step s7) and the vertical coordinate, for example, y-coordinate isused intact or is converted into a value obtained by subtracting thiscoordinate from the maximal value in the subject direction (step s8).Therefore, according to the present invention, both of the writingsurfaces can be used in the same manner regardless of consciousness ofrecognizing the writing surface.

According to this embodiment, the metallic member 94 and the non-contactsensor 95 and 96 form means for detecting the writing surface which isbeing used, while, the program and microprocessor 71 shown in FIG. 17form coordinate conversion means.

The sensing portion described in the first and second embodiments isusually formed by an insulating substrate having a printed conductivepattern as to correspond to the positions of the above-described x andy-direction loop coils. Alternatively, a structure may be employed whichis arranged such that a member in which a multiplicity of conductivewires are, at predetermined intervals, held between two insulating filmsand the thus-held conductive wires are connected to each other so as tocorrespond to the positions of the x and y-direction loop coils.

The most preferable example is described upon the electronic blackboardapparatus according to the present invention. For example, the loop coilfor generating the electric wave and the loop coil for detecting theelectric wave may be individually provided. In this case, the structuremay be arranged to always generate the electric wave.

Although the structure of the above-described embodiments is arranged insuch a manner that one sensing portion performs both the transmissionfunction and receiving function, a structure may be formed such that atransmitting sensing portion and a receiving sensing portion may beindividually provided. It is not necessarily critical for thetransmission and the reception to be subjected to the time-divisiontreatment. For example, a structure may be arranged such that thetransmission side continues the transmission and the reception sidedetects a predetermined electric wave from the transmission side byswitching only the coils of the tuning circuit.

In the foregoing, the present invention may be subject to variousarrangements, modifications and detailed changes in range that they donot deviate from the spirit. Therefore, the invention should not beunderstood within the limited meanings without adhering to the disclosedembodiment in the specification and drawings. The present invention isin the scope of the claims and further protected in the range that itagrees with the spirit.

What is claimed is:
 1. An electronic blackboard apparatus comprising:awriting surface, a tool for modifying an image on the surface includinga tuned circuit having a predetermined resonant frequency; means forsensing the presence of the tool relative to said surface, said meansincluding electric wave generating means for generating an electric wavewith plural frequencies, one of which is resonant to said frequency, andan electric wave detection means for detecting an electric wavereflected by said tuned circuit; said sensing means including coordinatedetection means responsive to the electric wave reflected from said tooland generated by said generating means for detecting a coordinatecorresponding to the position of said tool; and image informationprocessing means for processing, on the basis of the thus-detectedcoordinate, image information corresponding to an image formed by thetool on the surface.
 2. An electronic blackboard apparatus according toclaim 1, wherein said electric wave generating means and said electricwave detection means are arranged to be alternately operated.
 3. Anelectronic blackboard apparatus according to claim 1 or 2, wherein saidelectric wave generating means and said electric wave detection meansinclude x and y-direction loop coil groups.
 4. An electronic blackboardapparatus according to claim 1 or 2 further wherein a plurality of saidtools are provided, each of said tools having a tuned circuit with adifferent resonant frequency, equal to frequencies of the pluralfrequencies of the generated electric waves;the coordinate detectionmeans detecting the different resonant frequencies; writing toolidentification means for generating identification informationexpressing, on the basis of the detected frequency a characteristic ofsaid tool which is being used on said sensing portion; and imageinformation processing means for processing image information whichcorresponds to the image on said writing surface on the basis of thedetected coordinates of said tool and identification informationexpressing the characteristic of said tool which is being used.
 5. Anelectronic blackboard apparatus according to claim 1 or 2 wherein:a pairof said writing surfaces are disposed on opposite sides of said meansfor sensing, said writing surfaces being capable of being usedrepeatedly; means for detecting the surface which is being used fordetecting the writing surface which is being used; and coordinateconversion means for deriving indications in two coordinate directionsof said tool as derived from said coordinate detection means.
 6. Theapparatus of claim 5 wherein the two coordinate directions are in the xand y directions.
 7. The apparatus of claim 5 wherein the coordinateconversion means derives the indications intact.
 8. The apparatus ofclaim 5 wherein the coordinate conversion means derives the indicationsintact after the coordinates have been converted on the basis of thewriting surface which is being used.
 9. In combination, a positionsensing tablet; a two coordinate, direction coil arrangement associatedwith the tablet; a tool means adapted to be moved relative to the tabletand coil arrangement, the tool means including tuned circuits havingfirst and second resonant frequencies; means for supplying AC energy atthe first and second resonant frequencies to the coil arrangement; thetuned circuits on the tool means when the tool means is placed inproximity to the tablet and coil arrangement respectively causingchanges in currents flowing in the coil arrangement of said first andsecond frequencies; and means for sensing the changes in the currentflowing in the coil arrangement at said first and second frequencies andresponding to the current changes at said first and second frequenciesfor deriving a signal indicative of coordinates of the tool meansrelative to the tablet and coil means.
 10. The combination of claim 9wherein the energy at the different frequencies is sequentially suppliedto different coils of the coil arrangement.
 11. The combination of claim10 wherein the change is detected by supplying each of the coils duringa first interval with the AC energy at one of the predeterminedfrequencies, the means for sensing being activated to be responsive toenergy coupled back to each of the coils from the tuned circuit during asecond interval while the particular coil is not supplied with the ACenergy at the one predetermined frequency.
 12. The combination of claim9 wherein said tool means is cordless and no electric power supply isconnected to the tuned circuits thereof.
 13. The combination of claim 9wherein the signal is further indicative of first and secondcharacteristics of the tool means respectively associated with the firstand second frequencies.
 14. In combination, a position sensing tablet; atwo coordinate, direction coil arrangement associated with the tablet; atwo coordinate direction display superposed with the tablet; a markerfor the display; an eraser for the display; the marker including a tunedcircuit having a first resonant frequency; the eraser including a tunedcircuit having a second resonant frequency; means for supplying ACenergy at the first and second resonant frequencies to the coilarrangement; the tuned circuits on the marker and the eraser when themarker is placed in proximity to the tablet and coil arrangementrespectively causing changes in currents flowing in the coil arrangementof said first and second frequencies; and means for sensing the changesin the current flowing in the coil arrangement at said first and secondfrequencies and responding to the current changes at said first andsecond frequencies for denying a signal indicative of the position ofmarking by the marker on the display as modified by the eraser.
 15. Thecombination of claim 14 wherein the energy at the different frequenciesis sequentially supplied to different coils of the coils arrangement.16. The combination of claim 15 wherein the change is detected bysupplying each of the coils during a first interval with the AC energyat one of the predetermined frequencies, the means for sensing beingactivated to be responsive to energy coupled back to each of the coilsfrom the tuned circuit during a second interval while the particularcoil is not supplied with the AC energy at the one predeterminedfrequency.
 17. The combination of claim 16 further including another twocoordinate display responsive to the signal indicative of the positionof the modified markings for displaying the modified markings.
 18. Thecombination of claim 17 further including a plurality of said markers,each of said markers being for a different color and including a tunedcircuit with a different resonant frequency, the means for supplyingfeeding AC energy at each of the resonant frequencies to the coil, themeans for sensing responding to change sin the current flowing in saidcoil at each of the resonant frequencies, said another displaydisplaying the position and colors of markings made by each of saidmarkers.
 19. The combination of claim 16 wherein the means for supplyingsupplies different frequencies in sequence to the coil arrangement suchthat in response to the marker having the first resonant frequency beingproximate to and not being proximate the tablet the means for supplyingrespectively derives said first resonant frequency for first and seconddurations prior to deriving a second frequency, said first durationbeing considerably in excess of said second duration.
 20. Thecombination of claim 19 wherein each of the first durations is dividedinto a plurality of said first and second intervals.
 21. The combinationof claim 14 further including another two coordinate display responsiveto the signal indicative of the position of the modified markings fordisplaying the modified markings.
 22. The combination of claim 21further including a plurality of said markers, each of said markersbeing for a different color and including a tuned circuit with adifferent resonant frequency, the means for supplying feeding AC energyat each of the resonant frequencies to the coil, the means for sensingresponding to changes in the current flowing in said coil at each of theresonant frequencies, said another display displaying the position andcolors of markings made by each of said markers.
 23. The combination ofclaim 14, wherein said marker and eraser are cordless and no electricpower supply is connected to the tuned circuits thereof.
 24. Incombination, a position sensing tablet; a two coordinate direction coilarrangement associated with the tablet; multiple implements for movementrelative to the tablet, each of said implements having a differentcharacteristic associated therewith and including a tuned circuit havinga different resonant frequency; means for supplying AC energy at thedifferent resonant frequencies to the coil arrangement; the tunedcircuits on the implements when the implements are placed in proximityto the tablet and coil arrangement causing changes in the currentsflowing in the coil arrangement at said different frequencies; and meansfor sensing changes in current flowing in the coil arrangement at saiddifferent frequencies and responsive to the current changes at saidfrequencies for deriving a signal indicative of the position andcharacteristics of the implements relative to the tablet.
 25. Thecombination of claim 24 wherein the energy at the different frequenciesis sequentially supplied to coils of the coil arrangement.
 26. Thecombination of claim 25 wherein the change is detected by supplying eachof the coils during a first interval with the AC energy at one of thedifferent frequencies, the means for sensing being activated to beresponsive to energy coupled back to each of the coils from the tunedcircuit during a second interval while the particular coil is notsupplied with the AC energy at the one frequency.
 27. The combination ofclaim 26 wherein the means for supplying supplies different frequenciesin sequence to the coil arrangement such that in response to animplement having a first resonant frequency being proximate to and notbeing proximate to the tablet the means for supplying respectivelyderives said first resonant frequency for first and second durationsprior to deriving a second frequency, said first duration beingconsiderably in excess of said second duration.
 28. The combination ofclaim 27 wherein each of the first durations is divided into a pluralityof said first and second intervals.
 29. Apparatus for identifying acharacteristic and position of an implement having one of pluralcharacteristics, the implement including a tuned circuit having one ofplural different resonant frequencies, comprising a position sensingtablet; a two coordinate direction coil arrangement associated with thetablet; means for supplying AC energy at the plural different resonantfrequencies to the coil arrangement; the tuned circuit on the implementwhen placed in proximity to the tablet and coil arrangement causingchanges in current flowing in the coil arrangement at said onefrequency; means for sensing changes in the current flowing in the coilarrangement at said different frequencies and responsive to the currentchanges at said frequencies for deriving a signal indicative of theposition and characteristic of the implement on the tablet.
 30. Thecombination of claim 29 wherein the energy at the different frequenciesis sequentially supplied to coils of the coil arrangement.
 31. Thecombination of claim 30 wherein the change is detected by supplying eachof the coils during a first interval with the AC energy at one of thepredetermined frequencies, the means for sensing being activated to beresponsive to energy coupled back to each of the coils from the tunedcircuit during a second interval while the particular coil is notsupplied with the AC energy at the one predetermined frequency.
 32. Thecombination of claim 31 wherein the means for supplying suppliesdifferent frequencies in sequence to the coil arrangement such that inresponse to an implement having a first resonant frequency beingproximate to and not being proximate the tablet the means for supplyingrespectively derives said first resonant frequency for first and seconddurations prior to deriving a second frequency, said first durationbeing considerably in excess of said second duration.
 33. Thecombination of claim 32 wherein each of the first durations is dividedinto a plurality of said first and second intervals.
 34. The apparatusof claim 29 further including a two-coordinate direction displaysuperposed with the tablet adapted to be marked by the implement. 35.The apparatus of claim 34 further including another two-coordinatedirection display responsive to the signal for displaying the positionand characteristics of the implement on the tablet.
 36. The apparatus ofclaim 29 further including a two-coordinate direction display responsiveto the signal for displaying the position and characteristics of theimplement on the tablet.
 37. A method of identifying a characteristicand position of an implement on a position sensing tablet having a twocoordinate direction coil arrangement associated therewith, theimplement having one of plural characteristics and including a tunedcircuit having one of plural different resonant frequencies, comprisingsupplying AC energy at the plural different resonant frequencies to thecoil arrangement; the tuned circuit on the implement when placed inproximity to the tablet and coil arrangement causing changes in currentflowing in the coil arrangement at said one frequency; sensing changesin the current flowing in the coil arrangement at said differentfrequencies and responding to current changes at said frequencies forderiving a signal indicative of the position and characteristic of theimplement on the tablet.
 38. The method of claim 37 wherein the energyat the different frequencies is sequentially supplied to coils of thecoil arrangement.
 39. The method of claim 38 wherein the change isdetected by supplying each of the coils during a first interval with theAC energy at one of the predetermined frequencies, sensing energycoupled back to one of the coils from the tuned circuit during a secondinterval while the particular coil is not supplied with the AC energy atthe one predetermined frequency.
 40. The method of claim 39 wherein thedifferent frequencies are supplied in sequence to the coil arrangementsuch that in response to an implement having a first resonant frequencybeing on and not being on the tablet the first frequency is respectivelyderived for first and second durations prior to deriving a secondfrequency, said first duration being considerably in excess of saidsecond duration.
 41. The method of claim 40 wherein each of the firstdurations is divided into a plurality of said first and secondintervals.
 42. An eraser for supplying a signal to an electronic displayand for removing a marking from a surface of a visual display comprisinga housing, the housing including: a surface for erasing the marking,first and second tuned circuits each having a reactance positioned inproximity to first and second opposite edges of the erasing surface,first and second switches which when activated respectively cause thefirst and second tuned circuits to have different first and secondresonant frequencies, the first and second switches being positioned andarranged so that the first switch is activated in response to the firstedge of the eraser being pushed against the display surface and thesecond switch is activated in response to the second edge of the eraserbeing pushed against the display surface.
 43. The eraser of claim 42wherein the eraser is cordless and not electric power supply isconnected to the tuned circuits.
 44. In combination, a position sensingtablet; a two coordinate direction coil arrangement associated with thetablet: a two-coordinate direction display superposed with the tablet;multiple implements for modifying markings on the display, each of saidimplements having a different characteristic associated therewith andincluding a tuned circuit having a different resonant frequency for eachcharacteristic; means for supplying AC energy at the different resonantfrequencies to the coil arrangement; the tuned circuit on the implementswhen the implements are placed in proximity to the tablet and coilarrangement respectively causing changes in the currents flowing in thecoil arrangement at said different frequencies; and means for sensingchanges in the current flowing in the coil arrangement at said differentfrequencies and responsive to current changes at said frequencies forderiving a signal indicative of the position and characteristics ofmarkings by the implements on the display.
 45. The combination of claim44 wherein the energy at the different frequencies is sequentiallysupplied to different coils of the coil arrangement.
 46. The combinationof claim 45 wherein the changes are detected by supplying each of thecoils during a first interval with the AC energy at one of thepredetermined frequencies, the means for sensing being activated to beresponsive to energy coupled back to one of the coils from the tunedcircuit during a second interval while the particular coil is notsupplied with the AC energy at the one predetermined frequency.
 47. Thecombination of claim 46 further including another two coordinate displayresponsive to the signal indicative of the position of the markings andthe identification of the implements.
 48. The combination of claim 47wherein the implements include a plurality of markers, each of saidmarkers being for a different color, said another display displaying theposition and colors of the markings made by each of the markers on thefirst named display.
 49. The combination of claim 44 further includinganother two coordinate display responsive to the signal indicative ofthe position of the marking and the characteristics of the implements.50. The combination of claim 49 wherein the implements include aplurality of markers, each of said markers being for a different color,said another display displaying the position and colors of the markingsmade by each of the markers on the first named display.
 51. Thecombination of claim 44 wherein each of said implements is cordless andno electric power supply is connected to the tuned circuits thereof. 52.The combination of claim 50 wherein one of the implements is a markerand another of the implements is an eraser for markings of the marker,said signal being indicative of markings by the marker as modified bythe eraser, the another display being responsive to the signal toindicate the markings as modified by the eraser.
 53. The combination ofclaim 52 wherein the implements include a plurality of markers, each ofsaid markers being for a different color, said another displaydisplaying the position and colors of the markings made by each of themarkers on the first named display.
 54. In combination, a positionsensing tablet; a two coordinate coil arrangement associated with thetablet; a two coordinate direction display superposed with the tablet;plural markers for the display, each of the markers being for adifferent color on the display; each of the markers including a tunedcircuit having a different resonant frequency, means for supplying ACenergy at the different resonant frequencies to the coil arrangement,the tuned circuits on the markers when the markers are placed inproximity to the tablet and coil arrangement causing changes in currentsflowing in coils of the coil arrangement at said different frequencies;and means responsive to energy coupled between the tablet and markersfor deriving a signal indicative of the position of the colors ofmarkings by the markers on the display, said last named means sensingchanges in current flowing in the coil arrangement at said differentfrequencies.
 55. The combination of claim 54 wherein the energy at thedifferent frequencies is sequentially supplied to different coils of thecoil arrangement.
 56. The combination of claim 55 wherein the change isdetected by supplying each of the coils during a first interval with theAC energy at one of the predetermined frequencies, the means for sensingbeing activated to be responsive to energy coupled back to one of thecoils from the tuned circuit during a second interval while theparticular coil is not supplied with the AC energy at the onepredetermined frequency.
 57. The combination of claim 56 furtherincluding another two coordinate color-display responsive to the signalindicative of the position of markings for displaying the position andcolor of the markings.
 58. The combination of claim 54 further includinganother two coordinate color-display responsive to the signal indicativeof the position of markings for displaying the position and color of themarkings.
 59. The combination of claim 54 wherein said markers arecordless and no electric power supply is connected to the tuned circuitsthereof.
 60. A method of displaying a polychromic image comprisingmarking a first two coordinate direction display with plural markerseach having a different color and a tuned circuit with a differentresonant frequency thereon, applying plural frequencies to a twocoordinate direction coil arrangement, the coil arrangement coupling theplural frequencies to each marker as it is marking the first display,activating the coil arrangement so it is responsive to an interaction ofeach applied frequency and each tuned circuit to provide an indicationof the color and position of the mark being made by each marker on thefirst display, and displaying on a second two coordinate directiondisplay the position and color of the mark made by each marker on thefirst display by responding to the indication.
 61. The method of claim60 wherein each of the frequencies is sequentially applied to themarkers on the display.
 62. The method of claim 61 wherein thefrequencies are applied during a first interval and the response to theinteraction is derived during a second interval while the frequenciesare not applied.
 63. The method of claim 62 wherein the differentfrequencies are supplied in sequence to the coil arrangement such thatin response to an implement having a first resonant frequency being onand not being on the tablet said first resonant frequency isrespectively derived for first and second durations prior to deriving asecond frequency, said first duration being considerably in excess ofsaid second duration.
 64. The method of claim 63 wherein each of thefirst durations is divided into a plurality of said first and secondintervals.
 65. A method of indicating the position of an image on a twocoordinate direction display, comprising marking the display with amarker, erasing from the display at least a portion of marks made by themarker, said marker and eraser respectively having tuned circuits withfirst and second resonant frequencies, activating a two coordinatedirection coil arrangement so it applies said first and second resonantfrequencies to the marker and the eraser as they respectively mark anderase the display, and activating the coil arrangement to be responsiveto an interaction of the applied first and second frequencies with thetuned circuits to derive an indication of the position of the marking onthe display as modified by the eraser.
 66. The method of claim 65further comprising displaying the position of the marking on the displayas modified by the eraser by applying the indication to another twocoordinate direction display.
 67. The method of claim 66 wherein each ofthe frequencies is sequentially applied to the markers on the display.68. The method of claim 67 wherein the frequencies are applied during afirst interval and the response to the interaction is derived during asecond interval while the frequencies are not applied.
 69. The method ofclaim 68 wherein the different frequencies are supplied in sequence tothe coil arrangement such that in response to an implement having afirst resonant frequency being on and not being on the tablet, saidfirst resonant frequency is respectively derived for first and seconddurations prior to deriving a second frequency, said first durationbeing considered in excess of said second duration.
 70. The method ofclaim 69 wherein each of the first durations is divided into a pluralityof said first and second intervals.
 71. The method of claim 65 whereineach of the frequencies is sequentially applied to the markers on thedisplay.
 72. The method of claim 71 wherein the frequencies are appliedduring a first interval and a response to the interaction is derivedduring a second interval while the frequencies are not applied.
 73. Themethod of claim 72 wherein the different frequencies are supplied insequence to the coil arrangement such that in response to an implementhaving a first resonant frequency being on and not being on the tabletsaid resonant frequency is respectively derived for first and seconddurations prior to deriving a second frequency, said first durationbeing considered in excess of said second duration.
 74. An eraser forsupplying a signal to an electronic display and for removing a markingfrom a surface of a visual display comprising a housing, the housingincluding: a surface for erasing the marking, a tuned circuit having areactance positioned immediately behind the eraser surface, and switchmeans activated in response to the erasing surface being pressed againstthe display surface for connecting elements of the tuned circuit,including said reactance, together so they have a predetermined resonantfrequency while the erasing surface is pressed against the displaysurface.
 75. The eraser of claim 74 wherein the switch means includesfirst and second switches positioned and arranged so that the firstswitch is activated in response to a first edge of the eraser beingpushed against the display surface and the second switch is activated inresponse to a second edge of the eraser, opposite the first edge, beingpushed against the display surface.
 76. The eraser of claim 74 whereinthe eraser is cordless and no electric power supply is connected to thetuned circuit.
 77. The eraser of claim 74 wherein the reactance includesa coil having a longitudinal axis at a right angle relative to theerasing surface.
 78. In combination, a position sensing tablet; a twocoordinate direction coil arrangement associated with the tablet; a twocoordinate direction display; a first implement for causing marks toappear on the display; a second implement for causing marks to be erasedfrom the display; the first implement including a tuned circuit having afirst resonant frequency; the second implement including a tuned circuithaving a second resonant frequency; means for supplying AC energy at thefirst and second resonant frequencies to the coil arrangement; theimplements are placed in proximity to the tablet and coil arrangementrespectively causing changes in currents flowing in the coil arrangementof said first and second frequencies; and means for sensing the changesin the current flowing in the coil arrangement at said first and secondfrequencies and responding to the current changes at said first andsecond frequencies for deriving a signal indicative of the position ofmarkings by the first implement on the display as modified by the secondimplement.
 79. The combination of claim 78 wherein the display issuperposed with the tablet, the first implement and second implementbeing adapted to bear on the display.
 80. The combination of claim 78wherein the display is remote from the tablet and is responsive to thesignal to indicate the position of the markings as modified by thesecond implement.
 81. In combination, a position sensing tablet; a twocoordinate direction coil arrangement associated with the tablet;multiple implements adapted to be moved relative to the tablet, each ofsaid implements having a different characteristic associated therewithand including a tuned circuit having a different resonant frequency foreach characteristic; means for supplying AC energy at the differentresonant frequencies to the coil arrangement; the tuned circuits on theimplements when the implements are placed in proximity to the tablet andcoil arrangement respectively causing changes in the currents flowing inthe coil arrangement at said different frequencies; and means forsensing changes in the current flowing in the coil arrangement at saiddifferent frequencies and responsive to current changes at saidfrequencies for deriving a signal indicative of the position of theimplements relative to the tablet and of the characteristics of theimplements.
 82. In combination, a position sensing tablet; a twocoordinate coil arrangement associated with the tablet; a two coordinatedirection display; implement means for the display, said implement meansincluding plural tuned circuits each having a different resonantfrequency, a different resonant frequency being assigned to a differentcolor for the display, means for supplying AC energy at the differentresonant frequencies to the coil arrangement, the coil arrangementinteracting with the tuned circuits on the implement means when theimplement means is placed in proximity to the tablet to cause changes incurrents flowing in coils of the coil arrangement at said differentfrequencies; and means responsive to energy coupled between the tabletand the tuned circuits for deriving a signal indicative of the positionof the colors of marks on the display resulting from the different tunedcircuits being positioned relative to the tablet, said last named meanssensing changes in current flowing in the coil arrangement at saiddifferent frequencies.
 83. A method of displaying a polychromic imagecomprising moving implement means having tuned circuit means withresonant frequencies, each associated with a different color, relativeto a tablet having a two coordinate direction coil arrangement; applyingplural frequencies to the two coordinate direction coil arrangement, thecoil arrangement coupling the plural frequencies to the implement meansas it is moved relative to the tablet; activating the coil arrangementso it is responsive to an interaction of each applied frequency and eachtuned circuit resonant frequency to provide an indication of theposition and color assigned to the resonant frequency of the implementmeans relative to the tablet, and displaying on a two coordinatedirection display the position and color associated with the movement ofthe implement means relative to the tablet by responding to theindication.
 84. A method of indicating the position of the effectivemovement of a first implement that causes marks to appear on a twocoordinate direction display, the effective movement of the firstimplement being modified by a second implement that causes the marks tobe erased from the display, comprising moving the first and secondimplements relative to a tablet so that some of the area traversed bythe first implement relative to the tablet is subsequently traversed bythe second implement, said first and second implements respectivelyhaving tuned circuits with first and second resonant frequencies,activating a two coordinate direction coil arrangement of the tablet soit applies said first and second resonant frequencies to the first andsecond implements as they are respectively traversed relative to thetablet, and activating the coil arrangement to be responsive to aninteraction of the applied first and second frequencies with the tunedcircuits to derive an indication of the position of the marks on thedisplay as modified by the erasing action of the second implement. 85.An electronic blackboard apparatus according to claim 4 wherein:a pairof said writing surfaces are disposed on opposite sides of said meansfor sensing, said writing surfaces being capable of being usedrepeatedly; means for detecting the surface which is being used fordetecting the writing surface which is being used; and coordinateconversion means for deriving indications in two coordinate directionsof said tool as derived from said coordinate detection means. .Iadd. 86.An electronic writing tablet apparatus comprising:a writing surface, atool for modifying an image on the surface including a tuned circuithaving a resonant frequency; means for sensing the presence of the toolrelative to said writing surface, said means including electric wavegenerating means for generating an electric wave with plural frequencycomponents, one having a value equal to said resonant frequency, and anelectric wave detection means for detecting an electric wave reflectedby said tuned circuit; said sensing means including coordinate detectionmeans responsive to the electric wave reflected from said tool andgenerated by said generating means for detecting a coordinatecorresponding to the position of said tool..Iaddend..Iadd.
 87. Theelectronic tablet apparatus of claim 86 wherein said electric wavegenerating means and said electric wave detection means include x- andy-direction loop coil groups..Iaddend..Iadd.88. In combination, aposition sensing tablet; a two coordinate direction coil arrangementassociated with the tablet; a tool means adapted to be moved relative tothe tablet and coil arrangement, the tool means including tuned circuitmeans having first and second resonant frequencies; means for supplyingAC energy having first and second frequency components to the coilarrangement; the tuned circuit means on the tool means when the toolmeans is placed in proximity to the tablet and coil arrangement causinga change in a current flowing in the coil arrangement, and means forsensing the change in the current flowing in the coil arrangement andresponding to the current change for deriving a signal indicative ofcoordinates of the tool means relative to the tablet and coilmeans..Iaddend..Iadd.89. The combination of claim 88 wherein said toolmeans is cordless and no electric power supply is connected to the tunedcircuit means thereof..Iaddend..Iadd.90. The combination of claim 88wherein the first and second frequency components are respectively equalto the first and second resonant frequencies..Iaddend.